1. Field of the Invention
The present invention relates to a polar modulation transmitter and its transmission power control method.
2. Description of Related Art
FIG. 1 shows an example of a typical transmission apparatus using a polar modulation scheme. The transmission apparatus has polar signal generation circuit 1, amplitude control circuit 2, phase modulated signal generation circuit 3 and power amplifier (hereinafter “PA”) 4. In this transmission apparatus, the polar signal generation circuit 1 generates a signal from an input signal (i.e. a transmission modulated signal) signals related to the amplitude and phase of a transmission modulated signal. Amplitude control circuit 2 controls power supply voltage supplied to PA 4 based on amplitude component signals, and phase modulated signal generation circuit 3 generates phase modulated signals inputted to PA 4 based on phase component signals.
In practice, this transmission apparatus secures the dynamic range of transmission power by changing PA 4 between compressed mode and uncompressed mode. Further, compressed mode may be paraphrased as “saturated operation mode” and uncompressed mode as “non-saturated operation mode.”
This transmission apparatus operates PA 4 in compressed mode when high transmission power is required. On the other hand, the transmission apparatus operates PA 4 in uncompressed mode when low transmission power is required. To be more specific, in compressed mode, the transmission apparatus performs amplitude modulation by changing the power supply to PA 4 according to amplitude component signals. This compressed mode is inherently very accurate with respect to output power. On the other hand, in uncompressed mode, the transmission apparatus operates PA 4 in a less accurate condition than compressed mode with respect to output power.
However, with conventional transmission apparatuses, when compressed mode (“c-mode”) and uncompressed mode (“u-mode”) change in transmission power control, transmission power drift of maximum 5 dB or greater is likely to occur due to differences in characteristics between modes (i.e. drift due to temperature, drift due to wear, and drift due to load, etc.).
This will be explained briefly using FIG. 2. As shown in FIG. 2, output power in compressed mode is relatively accurate, but output power in uncompressed mode changes due to drift (i.e. drift due to temperature, drift due to wear, and drift due to load, etc.).
As shown in FIG. 2, output power in uncompressed mode is likely to drift due to various factors, and so, when compressed mode and uncompressed mode change, output power in uncompressed mode is likely to be discontinuous, and, as a result, significant drift in transmission power is likely to occur.
By the way, one method of performing transmission power control accurately is to measure the actual output power of a power amplifier and perform feedback control of output power such that this measurement value becomes equal to a set target value.
Generally, for this feedback control, the method of eliminating modulation drift components resulting from transmission data from output of the power amplifier using a low-pass filter, is employed. Then, transmission power is adjusted based on the difference between the set target value and the average transmission power which eliminates modulation drift components.
Here, more drift components such as modulated components can be eliminated by setting the time constant for the low-pass filter greater, so that it is possible to perform more accurate transmission power control.
On the other hand, if the time constant for a low-pass filter is set greater, the response of the low-pass filter becomes poorer, and, consequently, accompanying this, the responsivity of feedback control becomes poorer. There are wireless communication standards that require that transmission power control be completed in a very short period, and so, in practice, the time constant for the low-pass filter cannot be set greater significantly.
Particularly, when the above compressed mode and uncompressed mode change, feedback control with higher speed is needed, and so it becomes more difficult to make the time constant for the low-pass filter greater.
For example, according to 3GPP (3rd Generation Partnership Project) 25.101, differences in transmission power need to fulfill the requirements shown in FIG. 3 to FIG. 5.
This will be explained in detail. The Third Generation Partnership Project (3GPP), which is the standards body responsible for promulgating UMTS and W-CDMA standards, requires that TPC commands from a cellular network base station result in a mobile terminal increasing or decreasing its output power level in discrete steps (e.g., +/−1 dB, +/−2 dB, +/−3 dB, etc.). The UMTS standard also specifies that these power increasing and decreasing steps be performed within certain specified tolerances.
For example, as shown in the table of FIG. 3, in case of a TPC command for increasing and decreasing output power by a +/−1 dB step, resulting output power is required to be within +/−0.5 dB of target output power. Then, for example, if the transmission apparatus of a mobile terminal operates at output power 0 dBm and receives a TPC command for “1,” the transmission apparatus of the mobile terminal must adjust transmission power to be within the range between +0.5 dBm and 1.5 dBm. Wider tolerances of +/−1 dB and +/−1.5 dB are permitted for larger step sizes of 2 dB and 3 dB.
The 3GPP UMTS standard also imposes cumulative tolerances for groups of power commands, as shown in the table in FIG. 5. It is required that, for, for example, ten equal TPC commands of 1 dB step size each, the resulting output power level be within +/−2 dB of the target output power level.
As shown in the list of the table of FIG. 3 and FIG. 4, the most restrictive step size for a single TPC command is for a TPC command directing a+/−1 dB (+/−0.5 dB tolerance is required).
By the way, according to the 3GPP standard, power control is required to be performed within the range of ±25 μsec from the symbol boundary.
When compressed mode and uncompressed mode change, it is difficult for conventional transmission apparatuses to fulfill the above requirements.